1. Field of the Invention
The present invention relates to a motor-driven power steering system for automotive vehicles, and more particularly to a motor-driven power steering system which employs an electric motor for generating assistive steering power.
2. Description of the Relevant Art
Many automotive power steering systems employ hydraulic mechanisms or electric motors as assistive power sources. In power steering systems using electric motors, the assistive power source can be controlled directly by an electronic control unit (hereinafter referred to as an "ECU"). Such power steering systems are therefore more advantageous than power steering systems incorporating hydraulic mechanisms. Hydraulic power steering systems require a costly hydraulic control device or an additional mechanism which operatively interconnects an ECU and a hydraulic mechanism. An ECU may comprise a microcomputer, and such a control unit can be programmed to execute a complex control sequence. A typical motor-driven power steering system comprises a steering mechanism, an electric motor operatively coupled to the steering mechanism for generating assistive steering power, a power supply for supplying electric power to the motor, a motor driver connected to the power supply for driving the motor, and a controller for controlling the motor driver. The controller includes a microcomputer and a plurality of sensors connected to the microcomputer. The sensors detect basic factors such as steering torque acting on the steering shaft of an automobile and apply electric signals representative of the detected basic factors to the microcomputer. The microcomputer processes the supplied electric signals and issues a control signal for the motor driver.
It is advantageous if the electric motor is a DC motor and the motor driver is controlled by a chopper, particularly a pulse-width modulation (PWM) system, for controlling the current flowing through the DC motor. The PWM system can control the torque generated by the motor in a wide range since the motor is turned on and off in very short periods. In DC motor control, various motor drivers have been proposed and widely used for the purposes of changing the direction of rotation of the motor and controlling the motor torque through PWM systems. These purposes can well be served by a motor driver in the form of a bridge circuit, which will be described with respect to an embodiment and alternatives of the present invention. Such a motor driver has four terminals positioned respectively at the four corners of a rectangular network on a circuit diagram, with an element/elements connected between adjacent ones of the terminals. The paired terminals on a diagonal line are connected to the power supply, whereas the paired terminals on the other diagonal line are connected to the motor. The element/elements between adjacent terminals may be a combination of a diode and a switch which are connected parallel to each other, or may be a switching element with a diode incorporated therein. The diode/switch combination and the switching element are electrically equivalent to each other. The motor driver controls the PWM-driven motor such that it is repeatedly connected to and disconnected from the power supply at short periods through switching elements. Such disconnection between the motor and the power supply induces a considerable current in the motor. The diodes in the motor driver bridge form a loop which allows the induced current to flow therethrough, causing the opposite terminals of the motor to be substantially short-circuited. The loop will hereinafter be referred to as a "temporary short-circuiting loop".
The motor driver of the above arrangement has desired characteristics as a motor driver for a motor-driven power steering system. However, it has one disadvantage. Since the motor driver forms a temporary short-circuiting loop continuously during normal operation thereof, the "temporary" short-circuiting loop will remain present when the motor driver fails, and hence a closed loop which provides a short circuit between the input terminals of the motor tends to be formed continuously, but not temporarily.
Steering systems with assistive power sources are constructed on the principle of a fail-safe concept that the steering systems can manually be operated without power assistance should the assistive power sources fail to operate properly. When a power steering system with an electric motor is manually operated upon a power source failure, the output shaft of the motor is rotated by the steering effort of the automobile driver through a power transmission mechanism coupled to the motor output shaft, so that the motor operates as a generator. Where the "continuous" short-circuiting loop is formed at this time, a current is permitted to flow through the short-circuiting loop. Therefore, the motor functions as an electromagnetic brake, and some torque is required to rotate the motor. As a consequence, some additional steering effort is needed to manually operate the steering mechanism.